1. Field of the Invention
The present invention relates to a simple, convenient process for preparing high-resolution patterns of a conducting polymer on a substrate. The present invention also relates to the patterned conducting polymer surfaces so prepared and to devices, such as liquid crystal displays, which contain such a patterned conducting polymer surface. The patterned conducting polymer, e.g. polypyrrole, may serve as a conducting material to provide a means of addressing selected pixels of a lightweight, flexible liquid crystal display.
2. Discussion of the Background
In current liquid crystal (LC) display fabrication technology, the most widely used conductive material is indium tin oxide (ITO). However, the use of ITO has several disadvantages in manufacturing. One drawback is the relatively high temperature (about 250.degree. C.) at which ITO is deposited onto glass or other solid substrates, which can damage other components of the display such as polymeric color filters. Another problem is that when ITO is deposited on plastic substrates for purposes such as fabrication of flexible LC displays, the ITO becomes brittle and fails when the display is in a curved configuration for extended times. Thus, there is a need for a pliable conducting material to replace ITO for long-term operation of flexible and/or large area LC display devices.
An electrically conducting polymer is a desirable alternative to ITO as the conducting material, because it can be processed at ambient temperature and it is a flexible organic material similar to the plastic substrate. A general reference on the subject of conducting polymers is the monograph Organic Conductors, J. P. Farges, Ed., Marcel Dekker, NY, N.Y., 1994. The concept of using a conducting polymer as a replacement for ITO in a LC display is described in U.S. patent application Ser. No. 08/401,912, filed on Mar. 9, 1995.
The conducting polymer polypyrrole (PPy) is an excellent choice as a replacement conducting material for ITO, because PPy can be prepared by a convenient, in-situ polymerization method, and PPy films can be produced with surface resistance, electrical conductivity, and optical transparency characteristics that are appropriate for LC displays and other applications. The in situ method can be used to deposit thin, transparent films of electrically conducting polymers such as polyaniline and polypyrrole from aqueous solutions of the parent monomers, aniline and pyrrole, as they are undergoing polymerization. This method eliminates the necessity of first synthesizing the polymer and then dissolving it in a solvent for film deposition and is the simplest and least time-consuming approach for preparation of conducting polymer films. The use of aqueous solutions for the in-situ polymerization method also alleviates environmentally-related organic solvent disposal problems. Both of these factors are important for technological scale-up and commercialization of processes using conducting polymers.
PPy films deposited by the in-situ method can be prepared with conductivities &gt;300 S/cm using the anthraquinone-2-sulfonate counterion in the presence of 5-sulfosalicylic acid (see for example: R. V. Gregory, W. C. Kimbrell, and H. H. Kuhn, Synthetic Metals, vol. 28, p. C-823 (1989)). This translates to a surface resistance of between 600-3000 .OMEGA./square, nearly two orders of magnitude lower than that for typical polyaniline films deposited by the in situ method. These PPy films can also be obtained with optical transparencies in the range of &gt;65%-70% transparent. In-situ-deposited PPy films therefore have properties that make them very useful as conductive elements for the fabrication of a variety of liquid crystal display devices.
To fabricate an addressable LC display, one of the conducting elements must be patterned to a particular geometry and linewidths that constitute the designated output of the display. Depending on the nature of the LC display, the geometry may be in an alphanumeric pattern, a grid pattern, an array of dots, or some other pattern. The minimum dimension of the patterned features typically ranges from relatively large (&gt;100 .mu.m) to very fine (.about.10 .mu.m). A process for patterning conducting polymers must be able to meet the resolution requirements of the display. Additionally, the process for patterning the conductive polymer must not adversely affect the electrical surface resistance or the optical transparency of the conducting polymer in the active regions, or the properties of the underlying substrate. Finally, the patterning process must be simple, reproducible, cost-effective, and compatible with existing manufacturing equipment used in the LC display industry.
A number of methods have been reported for producing patterns of PPy on various substrates. In one method, long-chain alkyl self-assembled monolayer (SAM) films are stamped onto a gold substrate. The SAM-modified electrode is immersed in a solution of pyrrole, and the SAM film blocks electron transfer so that PPy is deposited only in the bare regions of the substrate. For a detailed description of this approach, see: C. B. Gorman, H. A. Biebuyck, and G. M. Whitesides, Chemistry of Materials, vol. 7, pp. 526-529 (1995). In related approaches, the patterned deposition of PPy can be initiated at semiconducting substrates by patternwise exposure of the substrate through a solution containing pyrrole monomer. The polymerization and deposition of PPy is then initiated photoelectrochemically at the substrate in the irradiated regions of the substrate. For a description of these approaches, see: M. Hikita, O. Niwa, A. Sugita, and T. Yamamura, Japan Journal of Applied Physics vol. 24, pp. L79 (1985); and M. Okano, I. Itoh, A. Fujishima, and K. Honda, Journal of the Electrochemical Society, vol. 134, p. 837 (1987). These methods are limited to metallic substrates such as gold or semiconducting substrates such as silicon, and are not appropriate for use on flexible, insulating polymeric substrates.
In a second method, a fluoropolymer substrate is modified by a plasma treatment to create regions of greater or lesser adhesion for a PPy film formed from an in-situ deposition method. The plasma is prevented from accessing the substrate in certain regions by the use of a physical mask such as a metal grid. The PPy is deposited initially over the entire substrate, but is then removed from the unmodified regions by either ultrasonication in solution or by removal with adhesive tape. For a description of these approaches, see: U.S. patent application Ser. No. 08/401,912 (see above) and L. S. van Dyke, C. J. Brumlik, W. Liang, J. Lei, C. R. Martin, Z. Yu, L. Li, and G. J. Collins, Synthetic Metals, vol. 62, pp. 75-81 (1994). These methods are limited to relatively low resolution PPy pattern formation, characterized by rough edges, and the use of physical masks is not compatible with standard manufacturing techniques for preparing commercially useful addressable liquid crystal displays.
In a third method, a PPy film deposited on a fluoropolymeric substrate was removed from selected regions of the substrate by laser ablation. For a description of this approach, see: L. S. van Dyke, C. J. Brumlik, C. R. Martin, Z. Yu, and G. J. Collins, Synthetic Metals, vol. 52, pp. 299-304 (1992). This approach is not preferred for practical applications due to the need for expensive, high-power laser exposure tools which are not desirable for cost-effective manufacturing, as well as the damage caused by laser ablation to the underlying substrate and the edges of the patterns.
In a fourth method, pyrrole vapor is absorbed into a film of a chlorine-containing polymer such as polychloroacrylonitrile (PCAN), and the polymer is then irradiated in a patternwise fashion. In the photolyzed regions, free radicals are created from dissociation of the chlorinated polymer, and the radicals initiate polymerization of the absorbed pyrrole monomer to form a PPy/PCAN composite. For details of this method, see: R. Baumann, K. Lennarz, and J. Bargon, Synthetic Metals, vol. 54, pp. 243-249, (1993). No electrical conductivity data for the polymerized pyrrole composite was reported. However, because this material is a mixture of PPy and an insulating material, the conductivity of the composite is almost certainly much lower than that of pure PPy which is a severe disadvantage for use of the composite as a conductive material for display fabrication. In addition, this process is limited to those chlorinated polymers that produce sufficient free radicals to initiate the polymerization of pyrrole. Of the four polymers tested, only PCAN required as little as 5 minutes of exposure with a high intensity (1000 W) lamp. Also, the reported times for sorption of pyrrole into the chlorinated polymer were .about.5-10 hours, which is impractical for use of this process in routine manufacturing.
Several methods have been described for patterning polyaniline (PAni). In one method, a film is prepared from a water-soluble derivative of PAni that contains a cross-linkable group on the polymer backbone. The film is exposed to electron-beam or UV irradiation, which crosslinks the polymer in the exposed region, and the unexposed areas are dissolved in water to produce the final patterns. Conductivities reported for the patterned PAni films produced by this method range from 10.sup.-2 to 10.sup.-5 S/cm, which are much lower than the value of 1 S/cm that is characteristic of the best films of unsubstituted PAni films (and again several hundred times worse than typical values for PPy films). For a description of this method, see: M. Angelopoulos, J. M. Shaw, N. C. Labianca, and S. A. Rishton, Journal of Vacuum Science and Technology, vol. B11, pp. 2794-2797 (1993). This method is limited to specially-derivatized PAni polymers, and the inherent requirement of derivatizing the PAni backbone with a cross-linkable group for pattern formation leads to severe degradation of the electrical properties of the conducting polymer, making it undesirable for use as an active element in LC display fabrication.
Thus, all of the above-described methods have inherent limitations with respect to their suitability for practical fabrication of high resolution patterns of highly conductive polymers on polymeric substrates. Thus, there remains a need for a process for preparing patterned conducting polymer surfaces which does not suffer from such drawbacks. In particular, there remains a need for a simple, convenient process for producing fine patterns of conducting polymers which exhibit good conductance and optical transparency.